The moonfish, or opah, is the first fish shown to be fully warm-blooded, researchers report in the journal Science. Although some large predatory fish, like tuna, can temporarily warm their muscles or organs, the opah is the only fish that warms its heart. A silvery fish the size of an automobile tire, the opah is found in oceans around the world.

To warm its organs, the fish harnesses the heat generated by rapidly flapping its pectoral fins. The heat gives the moonfish a competitive advantage in the chilly waters that it lives in, several hundred feet below the ocean’s surface.

Fruit flies appear to enter fearlike state if threatened

Whether fruit flies feel anything resembling emotion is hard to determine, but new research suggests they can experience a fearlike state.

“When a fly responds to a visual threat, it isn’t just a robotic reflex; there is some sort of internal state that develops,” said David J. Anderson, a biologist at the California Institute of Technology and an author of the new study, published in Current Biology.

The building blocks needed for emotion are called emotion primitives; these include persistence and scalability.

“If you’re hiking and hear a rattlesnake, your heart is going to pound and you experience fear long after the snake is gone,” Anderson said. “That’s persistence.”

Scalability refers to the gradation of an emotion: seeing 10 rattlesnakes will elicit more fear than seeing one.

Anderson and his colleagues put hungry fruit flies in a chamber with food and repeatedly passed a shadow over the food. The flies responded by jumping and running away.

After the last pass of the shadow, the flies continued to run, which the researchers said showed persistence. And as the number of shadow passes increased, the flies ran and jumped more quickly, suggesting scalability.

Despite these observations, it was not possible to conclude the flies were experiencing fear, Anderson said.

“We can only know that by verbal report,” he said. “So we can’t scientifically study feelings in any creature but a human.”